1. Field of the Invention
This invention relates to detecting cracks and structural integrity of materials using a remote wireless sensor.
2. Description of the Prior Art
Cracks in structural materials can be detected in many ways. Optical fibers are useful as strain gauges because their long length makes them amendable to detecting strain over wide areas [1]. Disadvantages of using optical fibers are that they are expensive to incorporate into the composite structure, and may additionally degrade material integrity [2]. Frequency response methods can be used to detect a change in stiffness of the structure by measuring a shift in natural frequencies when the structure has been damaged. Ultrasonic and X-Radiography are also used to characterize structural materials, but require access to both the front and back of the structure under test as well as the use of bulky equipment. Composite materials are not isotropic, making correlating results from acoustic detection methods difficult. All of these systems require a means to power the sensor and to physically connect to electrical leads to read out sensory information. In many cases, it is preferred that the sensor be built into the structural material with data read out wirelessly. Additionally, it is also preferred that the sensor can be powered wirelessly, eliminating the need for a power source connection or for an imbedded battery.
Radio-frequency identification RFID type communication systems have recently been integrated with strain gauges [3] to enable wireless transducers. These devices can be fabricated either monolithically with both a Micro-Electro-Mechanical System (MEMS) strain gauge and communications circuitry integrated on the same chip, or a hybrid approach using multiple IC's can be used to integrate the communications circuitry with the transducer. The hybrid approach requires more components and has potentially lower reliability. However, good performance is possible since the sensor and communications circuit fabrication can be done independently. The physical size of the antenna requires that it be implemented in a hybrid approach. In either implementation, these systems have a number of deficiencies, including large size, reduced reliability, and high cost.
In general, any system that must communicate through RF wireless communications requires the use of an antenna. In this invention, a device which serves the dual purpose of antenna and crack sensing gauge is described. The goal is to provide a very low cost sensing device that provides reliable material integrity information.